Circuit for minimizing transients during switching between two video channels



March 27, 1951 G. M. GLASFORD ETAL 2,546,338

CIRCUIT FOR MINIMIZING TRANSIENTS DURING SWITCHING BETWEEN TWO VIDEO CHANNELS 2 Sheets-Sheet 1 Filed May 13, 1947 March 27, 1951 E G. M. GLASFORD ETAL ,3 8

CIRCUIT FOR MINIMIZING TRANSIENTS DURING SWITCHING BETWEEN TWO VIDEO CHANNELS 2 Sheets-Sfieet 2 Filed May 13, 1947 m miu UNI HEOFDU Owl E 7.05m I! I 1 14/ vilvram Patented Mar. 27, 1951 CIRCUIT FOR MINIMIZING "TRANSIENTS DURING :SWITCHIN VIDEO CHANNELS Glenn M. Glasford, Passaic, and Edward M. Usher, Montc'lair, N. J., assignors to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a

corporation of Delaware G BETWEEN TWO Application May 13, 1947, Serial No. 747,3 2

Claims. 7 1 This invention relates to a device for selecting the video outputs of one or more television pickup tubes and provides means for automatically fading at a predetermined rate from one to the other or permitting lap dissolv of the two signals. Means are provided for eliminating any transients which might appear due to the switching operation. The signal with which this device is particularly useful is a television video signal after blanking has been added.

In accordance with this invention means are provided for varying at a predetermined rate the voltage at any electrode of a vacuum tube capable of controlling the electron flow within the tube between two voltages, one of which will permit the tube to operate normally, the other of which will not permit operation of the tube.

Transients are preventedjby a balanced clamping circuit that connects the grid of the tube that amplifies the composite signal to a fixed potential that may be ground every time a horizontal synchronizing pulse is applied to the input of this circuit.

The invention may be understood from the description in connection with the accompanying drawings in which:

Figs. '1 and 1a are diagrams of the circuit connections.

Fig. 2 is a graph of the grid voltages of the two controlled tubes when the time constants are the same, and

"Fig. '3 is a graph of the grid voltages when the time constants are not the same.

. In Fig. 1 reference character I indicates ,a cathode follower triode for channel I. The signal from the pick-up tube A applied to this channel is applied to the grid 2 of tube I through a capacitor 4 from the contact of the potentiometer 3 and across the resistor .5 connected back to the junction of two resistors l and '8 connected in series between ground and the cathode B of the tube The plate ll of the tube 1 is connected to a source of positive potential.

The resistor 8 in the cathode circuit of tube I is coupled through a movable contact 9 and a capacitor ID to the grid l3 of the tube [2. The screen grid [-4 is connected to a source of positive potential and the suppressor grid I5 is connected to cathode [6. The plate I1 is connected to the same source of positive potential through a resistor Ill. The cathode I 6 of this tube I2 is connected to ground through the capacitor l9 and resistor connected in parallel.

The output of the pickup tube '3 for channel H is connected to the grid '22 of cathode follower '2 t-riode 2| through the potentiometer 26 with contact point 25 and capacitor 24 across resistor 23 connected between the grid 22 and the juncture of two resistors 28 and '29 connected in series between the cathode 27 and ground. The resistor 29 is provided with a movable contact 38 which is coupled through capacitor 32 to the grid 34 of the pentode 33. The suppressor grid 3| is connected to the cathode 35. The plate 313 of this pentode '33 is connected to the plate l? of the pentode I2. The cathode 35 is connected to ground through the resistor 31 and capacitor 38 which are connected in parallel. The screen grid '39 is connected to a source of positive potential.

The control grid l 3 of pentode 12 is connected to ground through resistor 40 and capacitor 4| in series, and through variable-resistors 42 and $5 in series with a capacitor 44 connected between the juncture of resistors 42, 45 and ground. A single pole single throw switch 43 is connected across this capacitor 344. The other end of re- .sistor 45 is connected to a sliding contact 46 on resistor 4! that is connected between a source of negative potential and ground.

Control grid '34 of pentode 33 is connected to ground through a resistor 48 and a capacitor 49 which is also connected to a variable resistor 56 that is connected to one end of another variable resistor ,51 and to ground through a capacitor 52 that has a single pole single throw switch 53 connected across it. The other end of the resistor 51 is connected to the contact 46 on the resistor 4?.

The plates 11 and 36 of the pentodes i2 and 33 ,are connected together and to the grid 55 of the pentode 5'4 through a condenser 6%]. Grid 55 is returned to ground through resistor The cathode 59 'is connected to ground through a resistor Bi and condenser 81 in parallel, and also to the suppressor grid 51. The screen. grid 56 is connected to a source of positive potential that is connected to the plate 58 through a resistor 62.

Thegrid 63 of the pentode is connected to a source of synchronizing signals through condenser 63a. Resistors 63b and 530 provide means for placing a proper negative bias on grid 63. The cathode 65 of this tube 64 is connected to ground and to the suppressor grid 66. The screen grid 6:] is connected to a source of positive potential. The plate 68 is connected to this source of positive potential through the resistor 62 andis coupled to the grid 69 of the pentode it through the capacitor 15. The cathode H of this pentode is connected to the suppressor grid 13 and is also connected to ground through resistor 16, The output is taken 'ofi across this resistor 16. The screen grid 12 is connected to a source of positive potential and the plate 14 is also connected to a source of positive potential.

One end of one winding l1 (Fig. la) of a transformer i8 is connected to ground through a variable resistor 19 in series with a fixed resistor 80, the two resistors being connected in parallel with a capacitor 8 The other end of this winding 11 is connected to the grid 82 of a tube 83. The polarity of the windings is such as to cause this circuit to operate as a blocking oscillator. The plate 84 of this tube 83 is connected to one end of the other winding 35 of the transformer 18. The plate 84 of tube 33 is also connected to the plate 84' of tube 83. Grid 82 of this tube 83 is connected to a source of synchronizing signals, cathode 86' being grounded. The cathode 86 of tube 83 is connected to ground through resistors 8'! and 83, connected in parallel. Resistor 88 has a variable contact 39 that is connected to one side of capacitors 90 and 3|. The other side of capacitor 90 is connected to ground. The other side of capacitor 9| is connected to the junction of the plate 92 of diode 33 to one end of a resistor 94 and to the plate 95 of diode 96. The cathode 91' of the diode 33 is connected to the plate 98 of the diode 99 and to the grid 69 of the pentode 10 (Fig. l) by lead 69.

The cathode I33 of the diode 96 is connected to the plate lfli of the diode I02 and to a source of fixed potential which may be ground. The cathode |i33 of this diode I32 is connected to the cathode I34 of the diode 39 and the other end of the resistor 94 and to one side of the capacitor 535. The other side of this capacitor |35 is connected to the other end of the winding 85 of the transformer 18 and to the resistor m6. The other end of this resistor is connected through another resistor Hi? to a source of positive potential through a capacitor N58 to ground.

The operation is as follows:

When switch 43 is closed and switch 53 is open, the grid l3 of pentode I2 is returned to ground through resistors 43 and 42 and pentode l2 operates normally as an amplifier so that signals from channel I are amplified at the plate ll of this pentode.

The grid 34 of pentode 33 is returned to a negative voltage determined by the setting of the tap 43 on the resistor 4? which is normally set at a voltage below the cut-off point of pentode 33 so that no signal from channel II is then amplified by pentode 33 to appear at the plate 33.

When switch d3 is opened and switch 53 closed simultaneously, the voltage at the grid |3 of pentode i2 drops to the cut-off point in. a time determined by the double time constant 4|, 42 and 44, 45. As the pentode I2 is gradually out off by the building up of this negative grid voltage signals at the plate coming from channel I gradually fade out.

Simultaneously, the bias voltage at the grid 34 of pentode 33 gradually rises to ground level in a time determined by a single time constant comprising resistor 5e and capacitor 49, so that the signal coming from channel II-gradually appears at the plate 33 of the pentode 33.

The rate of fade out of one channel and the fade in of the other can be controlled by varying resistors 32, 53 and 5!. These resistors may be varied simultaneously. In practice, they may consistof ganged potentiometers or a number of resistors connected to a multi-position ganged switch. When all these resistors are zero, the switch-over is instantaneous, and when they are at maximum value, the time of switch-over is longest.

The variable resistor 41 controls the time between the disappearance of one signal and the appearance of the other. When the setting of contact 43 is near the least negative value the signals become mixed as they are switched. This is known as a lap dissolve. At a greater negative value one completely fades out as the other starts to come in. At a still greater negative value there is actually a short time between the fading out of signals from one pick up and the appearance of signals from the other one.

The reason for the double time constant 4|, 42 and 44, 45 or 43, 50 and 5|, 52 for cut-on" and a single time constant for turning them on is explained as follows:

If only one RC network were used in each grid circuit of tubes i2 and 33, the voltage at the grid l3 of pentode l2 would decrease at the rate indicated-by the graph a of Fig. 2 while the voltage at the grid 34 of pentode 33 would increase according to graph b of Fig. 2. With the cross-over of the two curves at cut-ofi for both tubes as shown in Fig. 2, one signal will disappear just as the other starts to come in, but the rate of rise of the grid potential of the second tube after the time at which the cross-over point occurs will not match the rate of fall of the grid potential of -ie first tube before the time of cross-over because of the exponential nature of the curves, i. c., the latter portion of one exponential with a given time constant will not match the beginning portion of the other exponential curve of a circuit with the same time constant.

By making the time constant of the two circuits unequal the effect of Fig. 3 can be achieved in which the two curves more nearly match so that the time one signal takes to fade out equals the time the other takes to come on.

The combined signal is applied to a stage of amplification comprising pentode 54. It is then mixed with the synchronizing signal that is applied to grid 63 or" pentode 34 and coupled to the grid 69 of pentode it through capacitor E5. Output is taken from the cathode of tube l3.

Diodes 33, 33, 93 and I02 are connected in a clamp circuit controlled by a driver circuit com.- prising transformer 73 and triode 83. Its function is to connect the grid 63 of pentode 13 to a fixed potential, which may be ground, every time a horizontal synchronizing pulse is applied to the circuit.

This arrangement permits the preceding amplifier stages to be designed for perfect response to the horizontal line and higher frequencies. It further eliminates any extraneous low frequency pickup or power supply ripple that may appear in the video channel. It also eliminates any transient output due to the operation of the channel selector switches.

The triode 83 and transformer '18 and the associated capacitors and resistors operate as a blocking oscillator. Negative pulses are obtained from the plate 84 through the winding 85 of the transformer l3 and the capacitor I35 to the juncture of resistor 94 and the cathode N4 of diode 99 and the cathode 13 of the diode H32. Positive pulses are obtained from the cathode 83 of the triode 83 across the resistor 81 and 88 at the variable contact 89 that is by-passed to ground through capacitor 93 and coupled through capacitor 9! to the juncture of plate 32 of diode 93 and plate 35 of diode 96 and one end of resistor 94. When I both a positive and a negative pulse appear at,

opposite ends of the resistor 94 all of the diodes 93, 96, 99 and I02 conduct thus connecting the grid 69 of pentode 10 to a fixed potential. All of the diodes are open except when these pulses appear.

The resistor 94 develops a voltage drop from any direct current leakage through the capacitors 9| and I05 and so tends to prevent either end of this resistor from reaching the potential of either the cathode 86 or the plate 84 of the triode 83. The purpose of this device is to hold the grid 69 of the pentode 10 at a constant potential regardless of transients and power supply ripple. With this device the system is required to pass only the higher frequency components of the signal. It also prevents the appearance of blemishes on the television picture when the switches are operated.

What is claimed is:

1. In a device for selecting the output signal from one of a plurality of television signal channels each said channel having an amplifier tube connected thereto, each said tube being normally conducting, a source of bias voltage, switching means connected to said source and to each said tube to apply selectively said bias voltage to each said tube to cut 01? said tube, and a resistance capacitance network connected to each said tube to control the rate of increase of said bias voltage applied thereto and the rate of decrease of said bias voltage removed therefrom by said switching means.

2. In a device for selecting the output signal from one of a pair of television signal channels each said channel having an amplifier tube connected thereto, each said amplifier tube being normally conducting, a source of biasing voltage and a pair of switches ganged together and connected to said source and to each said tube to apply said biasing voltage selectively to each said tube, and a resistance capacitanc network connected to each said amplifier tube, said switches being connected to change the time constant of each said resistance capacitance network to cause the time required by one said amplifier tube to out 01f when the switches are actuated and biasing voltage is applied thereto, to equal the time required by the other said amplifier tube to conduct when biasing voltage is removed therefrom.

3. A device for selecting video output comprising a plurality of television pickup devices each said pickup device having an electron discharge device connected thereto to amplify the video signal therefrom, each said electron discharge device having an electron beam controlling electrode, a source of bias voltage, an integrating circuit connected thereto, and a switching device connected to each said electrode to apply selectively said voltage thereto and also connected to said integrating circuit to change the time constant thereof.

4. The apparatus in accordance with claim 3 in which said integrating circuit comprises a resistance capacitance network and said switching device is connected to short out a portion of said network thereby to change the time constant thereof.

5. The apparatus according to claim 4 including a variable resistor connected between said source of voltage and said integrating network.

' GLENN M. GLASFORD.

EDWARD M. USHER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,043,997 Goldsmith June 16, 1936 2,164,297 Bedford June 27, 1939 2,240,420 Schnitzer Apr. 20, 1941 2,244,240 Blumlein June 3, 1941 2,307,375 Blumlein Jan. 5, 1943 2,412,279 Miller Jan. 5, 1943 

